A component is understood to mean a unit which provides a specific function. In this case, diodes, transistors, sensors can form a unit in order to provide mechanical and also electrical measurement objectives. Components can be provided in the form of electrodes in order to be able to be operated in a larger unit, such as an electrical circuit. A component can also be provided in the form of an integrated circuit on a chip. In this case, micromechanical structures can be formed, for example on a substrate which is defined with the aid of etching methods.
Nowadays, different types of micromechanical sensors, in particular inertial sensors, such as rate-of-rotation and acceleration sensors, are predominantly produced on different substrates. Consequently, different components are provided in order to achieve different measurement objectives.
In order to produce such electrical, mechanical or electromechanical components, use is made of methods and technologies which can typically be found in the semiconductor industry. One reason for separately producing the different sensors resides in the different physical requirements made of such sensors.
Consequently, different types of sensor are produced in different ways.
One type of sensor is a rate-of-rotation sensor, for example. Rate-of-rotation sensors are often encapsulated hermetically under a vacuum in order that they achieve high oscillation amplitudes as a result of subcritically damped resonant excitation, for example with a quality factor Q>0.5. However, a low electrical drive power is simultaneously provided in this case.
A further example of a type of sensor is an acceleration sensor. Acceleration sensors in comparison with rate-of-rotation sensors, by contrast, usually constitute a supercritically damped structure, for example with a quality factor Q<0.5. Acceleration sensors are typically constructed from mass oscillators, wherein the mass oscillators react to low-frequency accelerations. In this case, the relation between acceleration and mass deflection of an acceleration sensor should be linear to the greatest possible extent and have no resonant magnifications. In this case, a subcritical damping can also lead to incorrect measurements. This is the case if relatively high-frequency disturbances in the vicinity of the natural resonances of the acceleration pick-up act on the sensor, such that seismic masses of the acceleration sensor are excited in their resonant frequencies to carry out oscillations having large amplitudes. This can lead to high output signals which can no longer be adequately damped by output filters connected downstream.
Rate-of-rotation sensors and acceleration sensors are often required simultaneously for one measurement objective, for example in electronic stability control (ESP) or in roll-over protection systems in motor vehicles.
Various methods for providing these two types of sensor have been proposed heretofore.
One possibility is to provide an encapsulation of both structures of the rate-of-rotation sensor and of the acceleration sensor on a chip in a common cavity under a vacuum, wherein the acceleration sensor is subjected to greater damping artificially by electrical or signal processing methods.
This procedure has the disadvantages that structures in addition to the actual measurement transducer structures have to be integrated into the acceleration sensor, which increase the size of the sensor. Likewise, additional electronic circuit blocks have to be integrated into the signal processing (for example an application specific integrated circuit (ASIC)), which likewise increases the chip size of an integrated circuit (IC). In both cases, the production costs per chip are increased by the increased chip sizes. Consequently, the cost advantages of integration of both types of sensor on a chip are cancelled out or even turned negative.
A further possibility for providing different types of sensor is to provide an encapsulation of both sensor structures on a chip in a common cavity under defined pressure, wherein the acceleration sensor is supercritically damped. In this case, the required oscillation amplitude can be set by increased drive power and added drive structures within the rate-of-rotation sensor structure.
In the case of this procedure, as a result of the increased number of drive structures, the chip area of the rate-of-rotation sensor and the chip area of the ASIC are enlarged as a result of the provision of the higher drive power.
In order to provide a combination of two sensors, a further possibility is furthermore to provide an encapsulation of both sensor structures on a chip under a vacuum, wherein the acceleration sensor is accommodated in a cavity which is separate from the rate-of-rotation sensor and which has a ventilation opening. The ventilation opening is closed off hermetically for example after the encapsulation process under the pressure necessary for the acceleration sensor in a further method step.
In order to close off the ventilation opening under a pressure set in a targeted manner, homogeneous layer depositions that close off the entire wafer as a whole are known. This is provided for example with the use of sputtering or vapor depositions, such as chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), low pressure CVD (LPCVD), etc. However, for the reliable vacuum-tight closure of the ventilation opening, these methods require reduced pressure, which limits the maximum pressure to be set. Moreover, it is necessary to protect the encapsulated structure against the penetration of the layers into the cavity. Furthermore, stringent process engineering requirements are imposed during production in order that the closure of the ventilation opening is not prevented by particles or inadequate layer growth. Furthermore, the ventilation opening can also be closed off by the dispensing of closure material. However, this has the disadvantage that each chip has to be closed off individually and the closure materials have to be hermetically vacuum-tight.
Overall, the methods known heretofore are very cost-intensive.
It is an object of the present invention to provide at least two micromechanical structures in a component, in conjunction with cost-effective production of the component.
The object is achieved by means of the features of the components and methods in accordance with the present invention.